Can we now assume the role Rafe Spall originally auditioned for was David? I heard he originally auditioned for another character (undisclosed), but was chosen for Milburn. That just might be it.
This would imply him being Fifield, I guess.

Yeah, but I assumed the guy with Noomi here is Spall...maybe I'm mistaken and that's not him

All the spaghetti and goop is making me think this is an android.
Edit: No image showing up? Anyway, according to the Second Screen App trailer (see the Prometheus blu-ray preview thread), those shots were of screen tests. And that must have been Rafe Spall auditioning for David. I suppose that later on they were on the fence as to who would be the mutant - Milburn or Fifield, perhaps?